Finding the group from ionisation energies

7 minAS Level9.3.2

AI-drafted, independently checked · not yet teacher-reviewed

By the end of this lesson

By the end you can read a table of successive ionisation energies, find the big jump, count the outer electrons, name the group, and use the total number of ionisations to identify the element.

What you are counting

The first ionisation energy removes one electron from a gaseous atom. The second removes one electron from the 1+ ion made in the first step, and so on.

Each step removes an electron from the outermost occupied shell — until that shell is empty. The next electron then has to come from a shell closer to the nucleus, so it is much harder to remove.

That change shows up as a big jump in the values:

small, steady rises: electrons coming out of the same shell;
a large jump: the shell has been emptied and the next electron is coming from further in.

So the number of electrons removed before the jump is the number of electrons in the outer shell. That is the group.

From outer electrons to group number

Count the electrons before the jump, then convert:

Electrons before the jump	Outer electrons	Group
1	1	1
2	2	2
3	3	13
4	4	14
5	5	15
6	6	16
7	7	17
8	8	18

From Group 13 onwards the group number is ten more than the number of outer electrons. This is the single most common slip in these questions.

Worked example: Element X — jump after two

The successive ionisation energies of an element X are $738$ , $1451$ , $7733$ , $10543$ , $13630$ and $17995\ \text{kJ mol}^{-1}$ . Deduce its group.

Have a go first?

Check the method

Multiple choiceQuestion 1 · Recall

Answer the 2 questions above to continue.